J. Phys. Chem. C 2009, 113, 9737–9747
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Methane Activation by Nonthermal Plasma Generated Carbon Aerosols Nazim Muradov,* Franklyn Smith, and Gary Bokerman Florida Solar Energy Center, UniVersity of Central Florida, 1679 Clearlake Road, Cocoa, Florida 32922 ReceiVed: January 6, 2009; ReVised Manuscript ReceiVed: April 13, 2009
Activation of methane is one of the most challenging problems in catalysis due to the refractory nature of methane. Of particular interest is catalytic dissociation of methane as an attractive CO2-free route to production of hydrogen from natural gas. Synthesis, characterization, and evaluation of catalytic activity of plasmagenerated carbon aerosols for methane decomposition reaction are reported in this work. Carbon aerosols were produced by nonthermal plasma assisted decomposition of a carbon precursor gas (methane or propane) at near-ambient conditions. Plasma-generated carbons exhibited significantly higher catalytic activity for methane decomposition than known carbon-based catalysts with a comparable surface area. The mechanism of methane activation as well as the interrelation between the nanostructure of the plasma-generated carbons and their catalytic activity are discussed. The high catalytic activity of plasma-generated carbons for methane decomposition is attributed to the increased surface concentration of high-energy sites formed during nonequilibrium plasma assisted dissociation of a carbon precursor. 1. Introduction Efficient and environmentally friendly utilization of hydrocarbon resources, most importantly, methane or natural gas (NG), remains one of the cornerstone problems in chemical industry. The major technical challenge with methane conversion is that it is one of the most refractory organic molecules due to very strong C-H bonds (Edis ) 436 kJ/mol) and the lack of polarity. Over the past several decades, significant research efforts worldwide have been focused on the methane activation problem (see an excellent review paper on this subject1). Of particular practical interest is conversion of methane to hydrogen: one of the most important products in chemical and petrochemical industries. Due to its high gravimetric energy density, nonpolluting nature, and high chemical-to-electricity energy conversion efficiency, hydrogen is considered by many as a fuel of the future. Currently, most of the commercial hydrogen (amounting to about 80% of hydrogen produced in U.S.) is manufactured via the steam methane reforming (SMR) process. This process, however, produces substantial amounts of CO2 emission: a typical SMR plant with the capacity of one million standard cubic meters of hydrogen per day produces about 0.4 million m3/day of CO2, which is normally vented into the atmosphere. Single-step decomposition (or dissociation, cracking) of methane to hydrogen and carbon is considered2,3 a promising CO2-free alternative to SMR:
CH4 f C + 2H2
∆H° ) 75.6 kJ/mol
(1)
The process is moderately endothermic: the energy input requirement per mole of hydrogen produced (37.8 kJ/mol H2) is considerably less than that for the SMR process (63.3 kJ/ mol H2). Furthermore, in contrast to SMR, the process avoids the intermediate formation of CO and, hence, the need for a water gas shift reaction, which significantly simplifies the process. Finally, the process produces a value-added byproduct: clean carbon that can be utilized in a variety of traditional and * Corresponding author. E-mail:
[email protected]. Tel. +1-321638-1448. Fax: +1-321-504-3438.
novel application areas, thus, improving the process economics. Due to exceptional thermal stability of methane, the homogeneous (noncatalytic) methane decomposition process requires very high-temperatures (in excess of 1200-1300 °C). It should be noted that in the past, thermal noncatalytic decomposition of NG at temperature of 1400 °C has been practiced for commercial production of carbon black (with hydrogen byproduct being a supplemental fuel for the process).4 Recently, Kværner company (Norway) has developed a process for thermal (>5000 °C) plasma-assisted decomposition of NG to hydrogen and carbon black.5 An overview of different technological options for methane dissociation to hydrogen and carbon can be found in a recent publication.6 There have been extensive worldwide efforts to develop catalysts for methane decomposition reaction in order to reduce the maximum temperature of the process. Two types of catalysts for methane dissociation have been developed: metal- and carbon-based catalysts. It has long been known that certain transition metals (d-metals), most prominently, Fe, Co, Ni, and their alloys, exhibit high catalytic activity for decomposition of methane at moderate temperatures of 500-900 °C. However, the main problem with the metal-catalyzed methane decomposition reaction relates to rapid deactivation of the catalysts due to the blockage of their actives sites by carbon deposits. The use of carbon-based catalysts offers certain advantages over metal catalysts due to their durability, low cost, and sulfur resistance (which would allow utilizing commercial sulfurous hydrocarbon feedstocks without an expensive pretreatment stage). It has been reported that catalytic methane decomposition can be accomplished using high-surface-area carbons, such as activated carbons (AC) and carbon blacks (CB) at the temperature range typical of the SMR process (800-900 °C).7-9 AC showed higher initial catalytic activity than CB; however, the reaction was rapidly inhibited due to the blockage of the AC micropores by carbon deposits. In this work, a new type of carbon catalysts, namely, nonthermal plasma generated carbon aerosols are utilized for methane activation, which allows to significantly lower methane dissociation temperature (about 400 °C, relative to a noncatalytic
10.1021/jp900124b CCC: $40.75 2009 American Chemical Society Published on Web 05/06/2009
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Figure 1. Schematic diagram of the nonthermal plasma reactor for production of carbon aerosol particles. 1. A rotating electrode, 2. arc discharge, 3. a stationary electrode, 4. carbon aerosols, 5. a carbon collector, 6. a ceramic filter, 7. an electric motor, and 8. a high voltage power source.
reaction) and markedly increase methane conversion rate compared to other carbon-based catalysts. 2. Experimental Section 2.1. Reagents. Methane (99.99 vol %), propane (99.0 vol %), CO2 (99.9 vol %), and argon (99.999 vol %) were obtained from Air Products and Chemicals, Inc. and used without further purification. CABOT Corp. has provided the following carbon black (CB) samples: Regal 330 and Vulcan XC72 with the surface area of 94 and 254 m2/g, and the average particle size of 25 and 30 nm, respectively (the CB samples were used without further purification). Acetylene black (AB) (99.9+% purity, surface area 80 m2/g) was purchased from Alfa Aesar. Graphite powder (microcrystalline) (purity: 99.9995%, particle size: 2-15 µm) was obtained from Alfa Aesar. Metallic wires (1.0 and 1.6 mm in diameter) made of iron (99.99% purity), nickel (99.98%), Ni-Cu (45:55 wt %) alloy, stainless steel (Fe:Cr:Ni ) 70:19:11 wt %), as well as graphite rods (99.9995% purity, 3.05 mm in diameter) were obtained from Alfa Aesar and used as electrode materials in the nonthermal plasma reactor. 2.2. Apparatus and Procedure. Plasma-generated carbon aerosols were produced in an apparatus consisting of a nonthermal plasma reactor and a carbon collector as depicted in Figure 1. Nonthermal plasma was generated by the modification of a gliding arc (GlidArc) technique described by Czernichowski.10 A high voltage (12 kV) transformer was used as a power source for the generation of a nonthermal plasma arc-discharge. The plasma reactor with the volume of 2 L was made of Pyrex glass with an inlet for a carbon precursor gas (methane or propane) at the top and an outlet for the gaseous products and carbon aerosols at the bottom of the reactor. The lower stationary electrode was positioned vertically at the center of the reactor, and the upper electrode was rotating around the lower electrode at about 80 rotations/min (the circular motion of the upper electrode was necessary to avoid formation of carbon bridges between the electrodes and, thus, to ensure the smooth continuous operation of the plasma device). The distance between the electrodes was 0.7 and 2.5 cm in the narrow and wide points, respectively. Before the carbon generation experiments, the plasma reactor was purged by Ar for 0.5 h to remove air and moisture. A carbon precursor gas (methane or propane) was introduced into the plasma reactor at room temperature and atmospheric
Muradov et al. pressure at the flow rate varying in the range of 0.2-3.7 L/min. The carbon aerosol particles generated during high-voltage arc discharge between the electrodes were suspended in the lower section of the reactor, and eventually they were accumulated in a carbon collector (a certain portion of the carbon particles got stuck to the reactor wall, but they could be easily shaken off the wall by tapping). Gaseous products exited from the upper section of the carbon collector through a ceramic filter (designed to trap airborne carbon aerosols), and they were analyzed gas chromatographically. The yield of aromatic hydrocarbon byproduct: naphthalene was determined by extracting it with octane and quantifying by a UV-vis spectrophotometric method. Plasma-generated carbon (PGC) was removed from the carbon collector vessel at the end of the experiment and subsequently used as a catalyst in the methane decomposition experiments. The methane decomposition reaction over PGC was carried out in a fixed bed reactor (diameter of 10 or 12 mm) made out of quartz (to avoid a possible catalytic effect of the reactor wall on methane decomposition kinetics). A layer of the PGC (0.03 or 0.30 g) was placed between two thin layers of ceramic wool (the smaller amount of the PGC was used for determining the initial rates of methane decomposition, and the larger amount, in long-run experiments). Methane flow rate through the catalytic zone varied in the range of 60-160 mL/min-g (carbon). The reactor was maintained at a constant temperature using a type K thermocouple and a Love Controls PID temperature controller. All experiments were conducted at atmospheric pressure. Before the experiments, all PGC samples were heated at temperature of 500 °C for 0.5 h and at 800 °C for another 0.5 h in a stream of argon to remove surface-adsorbed species (oxygen, moisture, heavy organics). Methane was introduced to the reactor from the bottom section of the reactor, it passed over the PGC layer maintained at 850-900 °C, and the gaseous products of methane decomposition exited from the top of the reactor and entered a sampling loop of an in-line gas chromatograph. 2.3. Products Analysis and Carbon Characterization. Analysis of the gaseous products generated in the nonthermal plasma reactor and the methane decomposition reactor was performed gas-chromatographically using an SRI-8610A GC (thermal conductivity detector, argon carrier gas, silica gel packed column) and a Varian-3400 GC (flame ionization detector, helium carrier gas, HysepDB packed column). The surface area of the PGC samples was determined by a BET Sorptometer (BET201-A, Porous Materials Inc.). Raman spectra of carbon samples were recorded using Renishaw micro-Raman spectrometer equipped with 532 nm wavelength green laser (25mW) and 1800 lines grating (spectral resolution 1 cm-1). Inductively coupled argon plasma (ICAP) method was used for determining the trace quantities of iron and nickel in the PGC samples (method: EPA 6010). A thermogravimetric analyzer (Perkin-Elmer Pyris Diamond TG/DTA) was used for the measurements of CO2 gasification yields of different carbon samples. The surface morphology and microstructure of the PGC species were examined by scanning electron microscopy (SEM; JEOL 6400F) and transmission electron microscopy (TEM; Tecnai F30, 300 kV field emission source, equipped with STEM, HAADF detector and XEDS, manufacturer: FEI/Philips). Fourier transform infrared (FTIR) measurements of the PGC were performed using Perkin-Elmer Spectrum 100 spectrometer in the range of 650-4000 cm-1. X-ray diffraction (XRD) analysis of the PGC samples was conducted using Rigaku D-MaxB diffractometer. UV-vis spectrophotometric measurements of naphthalene were performed using UV-2401PC spectrophotometer (Shimadzu).
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Figure 2. Composition of gaseous products of nonthermal plasma assisted decomposition of methane as a carbon precursor gas. The plasma reactor is equipped with Ni electrodes.
3. Results and Discussion 3.1. Synthesis of Carbon Aerosols by Plasma-Assisted Decomposition of Carbon Precursors. Referring to the Figure 1, a stream of a carbon precursor gas (methane or propane) was introduced to the plasma reactor at room temperature. In the area between two electrodes (1), the carbon precursor gas was continuously exposed to an arc-discharge resulting in its decomposition to gaseous products and carbon aerosols (3) suspending and coalescing in the lower section of the reactor and, finally, accumulating in the form of fluffy carbon particles in a carbon collector (5) downstream of the plasma reactor. It is noteworthy that the carbon particles are so light that they readily become airborne (it is possible that they might carry some residual electrical charge). In a single pass, about 25-40% of the carbon precursor gas was converted to carbon aerosols and gaseous products. Figure 2 shows typical composition of the gaseous products of nonthermal plasma assisted decomposition of methane as a carbon precursor gas (unreacted methane was also included in the plot). It can be seen that hydrogen is a major gaseous product of methane conversion followed by acetylene and ethane-ethylene. A small amount of naphthalene was also detected on the surface of carbon particles (amounting to about 2 mol % of converted methane). For the comparison, the data on the gaseous products of thermal plasma assisted methane decomposition are also included in the Figure 2 (the data for the thermal plasma process are taken from ref 11, N2 is not included). Evidently, in the thermal plasma process, the methane conversion yield and, consequently, hydrogen concentration in the product mix, are higher than those in the nonthermal plasma process, which can be attributed to much higher temperatures in the thermal plasma (interestingly, acetylene concentrations in both cases are comparable). Nonthermal plasma assisted decomposition of propane as a carbon precursor gas also produced hydrogen as a main gaseous product, but the process also generated much higher yields of C2+ hydrocarbons and naphthalene compared to methane. The surface area of the PGC samples produced with the aid of different electrode materials in the plasma reactor were determined by a standard BET method, and they are presented in the Table 1. The surface areas of the PGC were found to be within a relatively narrow range of values: 129-150 m2/g (which is greater than the margin of error of about 5%). At this point, it is difficult to decipher which factors affect the surface area of the PGC, and whether the nature of the electrode material has any effect at all. It can be speculated that slight deviations in
no.
carbon precursor
nonthermal plasma reactor electrode material
nonthermal plasma
1 2 3 4 5 6 7 8
CH4 CH4 CH4 CH4 CH4 C 3 H8 CH4 C2H4
graphite rod Ni wire stainless steel (SS) wire Fe wire Ni-Cu wire graphite rod graphite rod graphite rod
130 132 150 147 129 138
thermal plasma
50-69 62-88
the arc electric current, or residence time, or local temperature could potentially affect the surface area of the PGC. For the comparison, the data on the surface area of thermal plasma generated carbons (taken from ref 12) are also included in the Table 1. It can be seen that the surface area of the thermal plasma generated carbons is significantly less than that of carbons produced via nonthermal plasma assisted processes (according to the authors of the study,12 the relatively low surface area of the thermal plasma carbons can be attributed to lack of quenching that controls the carbon particle size). In the case of nonthermal plasma generated carbons, the quenching is not necessary due to nonequilibrium conditions in the plasma. The further discussion in this paper is limited to the nonthermal plasma generated carbons produced from methane as a carbon precursor gas. 3.2. Methane Decomposition over Plasma Generated Carbons. Undiluted methane was introduced to the methane decomposition reactor at the atmospheric pressure and decomposed over the PGC layer at 800-900 °C into carbon (which was deposited on the PGC) and the gaseous products exiting the reactor. Control experiments using inert contacts (e.g., quartz powder and ceramic wool) indicated that no appreciable thermal (i.e., noncatalytic) decomposition of methane occurred at these experimental conditions. The carbon product yield was quantified by subtracting the weight of original PGC from the weight of carbon in the reactor after the methane decomposition experiment. A series of methane decomposition experiments was conducted using PGC produced in the plasma reactor equipped with the electrodes made of different materials, such as graphite, Ni, Fe, SS, and Ni-Cu. The results are presented in Figure 3, panels A and B, which depicts the initial methane decomposition rate per unit of carbon weight (A) and per unit of carbon surface area (B) (reaction temperature 890 °C). For the sake of comparison, Figure 3 includes the data on the initial rate of methane decomposition over other types of amorphous carbon with comparable surface area, namely, acetylene black and two forms of carbon black: Regal 330 and Vulcan XC72 (note that the PGC surface area is roughly a half way between that of Regal 330 and Vulcan XC72). It can be seen from Figure 3 that the initial rates of methane decomposition over the PGC samples are significantly higher than that of AB and both carbon blacks. The activation energies determined from the initial rates of methane decomposition over PGC produced with the aid of SS and Ni electrodes were Ea) 165 and 172 kJ/mol, respectively (which are somewhat lower than the Ea values reported for carbon blacks13). Figure 4A depicts the kinetic curve of methane decomposition over PGC at 890 °C during a prolonged (37 h) experiment. The
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Figure 3. Comparison of initial rates of methane decomposition at 890 °C using nonthermal plasma generated carbons, carbon blacks and acetylene black. The comparisons are based on per unit carbon weight (A) and surface area (B) basis. Carbon black: 1, acetylene black; 2, carbon black Regal 330; 3, carbon black Vulcan XC72; 4-8, nonthermal plasma generated carbons using graphite (4), Ni (5), stainless steel (6), Fe (7), and Ni-Cu (8) electrodes, respectively.
plasma-generated carbon was produced from methane as a carbon precursor in the plasma reactor using Ni electrodes. During the first half hour, the rate of methane decomposition sharply dropped by about a quarter of its initial value and remained at that level for about 20 h. As reported earlier,7 carbon blacks also showed an initial drop in methane decomposition rate, but they exhibited significantly lower catalytic activity for methane decomposition than PGC. Figure 4B depicts the time dependence of the distribution of gaseous products of methane decomposition over the same type of PGC (as shown in Figure 4A) and at the same experimental conditions (for simplicity, unreacted methane is not shown on the plot). Ethylene and ethane were produced in small quantities (below 1 vol %), although ethylene concentration slowly increased over the experimental run. The trace quantities ( dGraph). Although graphite is thermodynamically the most stable form of carbon (at standard conditions), carbon produced by PGC-catalyzed methane decomposition tends to arrange itself into a turbostratic structure since it is energetically and kinetically the most preferred pathway at given temperature (850-900 °C) and pressure (1 atm). To convert turbostratic carbon to graphite it has to be heated to temperatures in excess of 2500-3000 °C.
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Figure 9. X-ray diffractogram of different carbon samples: A, plasma-generated carbon (stainless steel electrodes, carbon precursor: methane); B, plasma-generated carbon exposed to methane at 890 °C for 2 h; C, graphite powder.
Figure 10. Raman spectra of carbon samples: 1, plasma-generated carbon (stainless steel electrodes, carbon precursor: methane); 2, plasmagenerated carbon exposed to methane at 890 °C for 8 h.
3.4. Interrelation Between Structure of Plasma-Generated Carbons and Their Catalytic Activity. 3.4.1. Nanostructure of Plasma-Generated Carbons. The results of analyses and characterization of the PGC samples indicate that their structure
have certain similarities and differences with that of commercial carbon blacks. It is known that carbon blacks consist of hexagon carbon layers, and the carbon atoms within each layer are arranged in almost the same manner as in graphite (C-C distance within the layer is 1.420 Å).14 The relative position of these layers, however, is random, so that there is no order in columnar (stacking) direction. Different forms of carbon black differ in the size of these crystalline regions (or crystallites) from a few tens to hundreds Å.17 The fraction of “crystalline” carbon in carbon blacks varies in a wide range of 60-90%.14 This explains the appearance of characteristic (though, not very strong) lattice fringe (002) reflections in the X-ray diffractogram of many carbon blacks (see, for example, the diffractogram of carbon black Black Pearl 2000 reported in our earlier work8). Thermo-gravimetric measurements using CO2 as a gasifying agent were conducted in this work to estimate the fractions of disordered and quasi-crystalline carbon in the PGC samples (that were produced with the aid of graphite, SS, Ni, and Ni-Cu electrodes). It should be noted that carbon gasification and oxidation reactions are widely used to determine the fractions of amorphous and crystalline carbons in the carbon samples (which is based on different reactivity of two forms of carbon).14 It was found that between 52 and 66 wt % of PGC (depending on the sample) was gasified by CO2 before the temperature reached 1000 °C. For comparison, carbon black Vulcan XC72 with the comparable surface area lost only 15-20% of weight during CO2 gasification reaction at comparable conditions. Although it is difficult to quantify the percentage of amorphous and nanocrystalline carbons in the PGC and Vulcan XC72 samples from these experiments, it is evident that the fraction of amorphous (more reactive) carbon in the PGC is considerably higher than that in the carbon black sample. This observation,
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coupled with the results of XRD and TEM analyses indicate that PGC is structurally less ordered than conventional carbon blacks. Why do different forms of amorphous carbon such as PGC, CB, and AB have different degree of structural ordering? It is well-known that heat treatment significantly alters the microstructure of carbon blacks by enhancing their structural order, in particular, via increasing the average diameter of the carbon crystallites and reducing the distance between the graphene layers. It was reported, for example, that heating carbon black just over 1000 °C markedly affects its structure, and at the temperature of about 1800 °C the average carbon crystallite size (La) increases from original La ) 4.0 to 7.0 nm, and the interlayer distance (d002) in the carbon black is reduced from original d002 ) 3.55 to 3.41 Å.18 This implies that the method and operational conditions of manufacturing of amorphous carbons markedly affect their microstructure. Most of commercial carbon blacks are produced either by pyrolysis of light hydrocarbons (CH4, C2H2) or partial combustion of heavy hydrocarbon feedstocks at very high temperatures of 1200-1800 °C.14 As a result, a certain fraction of the carbon layers in carbon black could be rearranged to a graphite-like order at temperatures above 1200 °C, beginning at the particle surface (at 2500-3000 °C, graphite crystallites are formed and the carbon black particles assume polyhedral shape14). This explains a certain degree of structural ordering in commercial carbon blacks. In contrast, nonthermal plasma-generated carbons are formed at near ambient temperature and pressure conditions. Nonthermal plasma is characterized by very high electron temperatures, but low bulk gas temperatures (i.e., velocity distribution of the species does not follow a Maxwell-Boltzmann distribution, therefore, such plasma is also called “non-equilibrium” plasma). According to literature sources, electronically excited methane molecules (CH4*) play a major role in nonthermal plasma assisted conversion of methane to hydrogen and carbon. In particular, Oumgar et al.19 reported that during nonthermal (corona discharge) plasma initiated decomposition of methane, CH4* is a precursor of a number of CHx radicals leading to formation of carbon and hydrogen as follows:
If one accepts this mechanism, formation of C2H6, C2H4 and C2H2 during nonthermal plasma assisted methane decomposition could be attributed to dimerization reactions involving corresponding radicals (CH3, CH2 and CH, respectively), for example:
2CH3 f C2H6
(4)
In an alternative mechanism, after the initial formation of C2H6, other products could be formed via a stepwise dehydrogenation route (similar to a conventional thermal route20):
C2H6 f C2H4 f C2H2 f aromatics f polynuclear aromatics f carbon (5) Naphthalene detected among the products of plasma-assisted conversion of methane, in all likelihood, is a secondary product
formed by condensation of acetylene. The catalytic role of the plasma-generated carbon aerosols in the above reactions cannot be excluded. Due to pronounced nonequilibrium conditions in nonthermal plasma combined with a near ambient temperature surrounding, the carbon product generated from a carbon precursor acquires a highly disordered structure, and, most importantly, it would not undergo the structural ordering process to form thermodynamically more favorable graphite-like structures (as in thermally produced carbons). As a result, different forms of C-C bonds may coexist in the plasma-generated carbons, including sp3, sp2, and mixed hybridized carbons (and, possibly, sp carbons). This assumption is supported by the results of FTIR spectroscopy, which indicates the presence of the vibrations corresponding to nonaromatic C-C bonds. The results of Raman spectroscopy also confirms the presence of sp3C-sp3C and mixed sp2C-sp3C bonds in the PGC (Figure 10). It should be noted that similar observations with regard to the existence of sp3C-sp3C and sp2C-sp3C bonds in carbons produced by means of high-energy sources (laser, arc-discharge) have been reported in the literature.21,22 In particular, one study reported that amorphous carbon films produced by plasma and electric-field assisted processing of carbon-containing gases contain mainly sp2 carbon, but their sp3 content can be varied over the range of 5-55%.15 Based on the results of the structural and spectroscopic analyses, it is possible to envision the PGC nanostructure with the areas of sp2, sp3 carbon hybridization, and mixed sp2-sp3 bonds, as schematically shown in the Figure 12. On the surface of these carbon species, the regular array of C-C bonds is disrupted, forming free valencies, discontinuities, dangling bonds and other energetic abnormalities that can be generalized as high-energy sites (HES). The presence of these surface abnormalities could further distort the carbon nanostructure; in particular, it has been pointed out that amorphous carbons with high concentration of dangling bonds are characterized by deviations in C-C interatomic spacing of more than 5%, and noticeable variations in the bond angle.23 It can be hypothesized that, due to the different nature of C-C bonds (i.e., sp2 and sp3) and degree of nanocrystallinity existing in the PGC, several types of HES can exist on the PGC surface: those are distinguished by the bond energetics (i.e., their relative reactivity toward methane) and their surface concentration. Although the data on the relative strength of different C-C bonds may provide some clue with regard to the relative reactivity of the corresponding HES (e.g., the C-C bond strength within graphene layers is 452 kJ/mol, compared to 347 kJ/mol in the sp3C-sp3C structure),24 at this point, it is difficult to single out the certain type of high-energy sites that is predominantly responsible for the activation of methane molecules. The TEM image of PGC presented in Figure 7 (inset B) provides a view of several carbon nanocrystallites with exposed edges, which, presumably, represent the areas with high concentration of HES. Evidently, due to the highly disordered structure of PGC the surface concentration of the HES in PGC is much greater than that in carbon blacks with a relatively ordered structure, which explains the difference in their catalytic activity for methane decomposition (when comparing PGC and CB with comparable surface area). 3.4.2. Proposed Mechanism of Methane Decomposition oWer Plasma-Generated Carbons. The mechanism of the PGCcatalyzed methane decomposition reaction at a molecular level is very complex and yet to be fully understood. In all likelihood, methane activation occurs on the surface of PGC via interaction
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Figure 11. Schematic representation of turbostratic (A) and graphitic (B) carbons. Upper and lower drawings correspond to the top and crosssectional views, respectively.
Figure 12. Hypothetical structure of nonthermal plasma generated carbons.
of methane with the high-energy sites (e.g., dangling bonds) functioning as active sites. It is presumed that methane first reacts with the HES having highest reactivity, followed by the sites with lesser reactivity. This assumption allows the shape of the kinetic curves of methane decomposition to be explained, in particular, the sharp drop during the first hour followed by a quasi-steady state process (Figure 4). In the proposed mechanism of methane dissociation, methane molecules interact via dissociative adsorption with chemically reactive HES on the surface of PGC:
CH4 f CH3-*CPG + H-*CPG
(6)
where *CPG denotes an active site on the surface of plasmagenerated carbon. Breaking of the first C-H bond is likely to be the rate-limiting step of the methane decomposition reaction. Using first principles calculations at the density functional theory level, Huang et al. estimated the energetics of the interaction between methane molecules and carbon atoms at the graphene edge.25 In particular, the authors determined that the activation barrier for methane dissociation over a graphene edge with dangling bonds is 77.3 kJ/mol. Figure 13 depicts the schematics of methane activation via the dissociation of the first C-H bond over a graphene edge acting as HES (based on the activation barrier data presented in ref 25).
The above value of activation energy is rather close to the activation energy of a transition metal-catalyzed methane dissociation reaction (which is about 60 kJ/mol26). Thus, potentially, the ability of carbon HES to activate methane is energetically comparable to that of known metal catalysts (e.g., Ni, Fe, and Co). The experimentally determined activation energies for methane dissociation over PGC differ from the above theoretical values, which may point to a difference in the nature of active sites or the mechanism of C-H bond activation. The intermediate formation of methylated derivatives on the surface of fullerene soot exposed to methane at high temperatures (as reported in ref 27) indirectly points to the possibility of the reaction (6) (the analogy here is that fullerene soot exhibits structural irregularities, and, thus, relatively high surface concentration of HES). The initial methane activation stage is followed by a series of stepwise dissociation reactions leading to carbon and hydrogen:
CH3-x-*CPG f CH2-x-*CPG + H-*CPG H-*CPG f
1 H + *CPG 2 2
(7) (8)
where 0 < x < 2. During this process, C-H bonds in methane molecule break and new C-C bonds in graphene layers of carbon form. It is
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Figure 13. Schematics of methane activation via the dissociation of the first C-H bond over a graphene edge.
Figure 14. Schematic representation of BSU and LMO structures as carbon growth sites on the surface of plasma generated carbons.
hypothesized that the carbon build-up over the PGC surface is a combination of two simultaneous events: carbon nuclei formation (route B) and carbon crystallites growth (route A):
where CPG and Cm are plasma generated carbon and carbon formed by decomposition of methane, respectively, and CSNC, CCN, and CTS are subnanocrystalline, nanocrystalline, and turbostratic forms of carbon, respectively. According to Oberlin, carbon is composed of elemental bricks called a basic structural unit (BSU) defined as a parallel stack of two to four carbon layers each containing less than 10-20 aromatic rings.28 An array of several BSU with a near common orientation forms a local molecular ordering (LMO) structure, as shown in Figure 14. The notion of existence of BSU and LMO can be useful from the viewpoint of the nucleation and growth mechanism discussed here. In particular, as it follows from the foregoing PGC analyses and characterization data, PGC exhibits a predominantly disordered structure with some nanocrystalline inclusions (see, e.g., the TEM image in Figure 7, inset B). These carbon nanocrystallites (which may be viewed as LMO) could provide a template for the layer-by-layer build-up of carbon leading to relatively large carbon crystallites and, ultimately, to carbon with a typical turbostratic structure (route A). Partially ordered turbostratic carbon has relatively low surface concentration of HES, and consequently, low catalytic activity for methane decomposition reaction (thus, the reaction via the route A is inhibited).
Muradov et al. The nucleation process, in all likelihood, involves sp3 and mixed sp3-sp2 carbon sites that give rise to subnanocrystalline carbon (CSNC) structures as the first step in carbon build-up on PGC. The subnanocrystalline carbon consists of one or several BSU (or, possibly, LMO), and they act as nuclei for the growth of carbon nanocrystallites (CNC) (route B). The formation of the nuclei as well as active growth sites at the “edges” of the BSU and LMO nanostructures promotes the carbon growth and, consequently, the methane decomposition reaction. The proposed nucleation-growth mechanism allows explaining the difference in the kinetics of methane decomposition over various forms of carbons. Due to well-ordered structure of graphite, methane decomposition over microcrystalline graphite proceeds almost exclusively along the route A, but since the surface concentration of HES is very low, the catalytic activity of the graphite powder is extremely low. In the case of carbon blacks, both A and B routes contribute commensurate with their degree of structural ordering (the high concentration of nanocrystallites in CB favors route A). Acetylene black is structurally more ordered than other forms of carbon blacks,29 and consequently, it is less catalytically active in methane decomposition reaction compared to CB Regal 330 with a comparable surface area (see Figure 3). PGC is the least structurally ordered carbon among tested amorphous carbons, thus, during the initial stage of the process methane decomposition proceeds preferably along the nucleation route B, followed by increasingly greater contribution of the crystallite growth route A. 4. Conclusions Catalytic methane decomposition is an attractive CO2-free route to production of two valuable products: hydrogen and carbon. However, the development of active catalysts for methane decomposition presents a significant challenge due to an exceptional stability of methane, which hinders the practical implementation of the process. The results of this work demonstrate that nonthermal plasma generated carbon aerosols are capable of activating methane and catalyzing its dissociation at temperatures about 400 °C lower than those of the thermal (noncatalytic) reaction. Due to nonequilibrium conditions in nonthermal plasma (i.e., high electron temperature and low bulk gas temperature), the carbon product formed by decomposition of a carbon precursor exhibits structural characteristics dissimilar to that of other forms of amorphous carbon, e.g., carbon blacks. The results of analyses and characterization of plasma generated carbons using TEM, XRD, Raman spectroscopy, and TGA methods are indicative of a highly disordered structure of PGC, more disordered than that of carbon black, acetylene black and other amorphous carbons. The PGC structural characterization data suggest that different forms of C-C bonds may coexist in these carbons, including sp3 and sp2 hybridized carbons along with mixed sp3C-sp2C bonds. On the surface of these carbon species, the regular array of C-C bonds is disrupted, forming dangling bonds, surface radicals and other energetic abnormalities that function as active sites during activation and subsequent dissociation of methane molecules. It is proposed that carbon build-up on the surface of plasma-generated carbons is a combination of two simultaneous events: carbon nuclei formation and carbon crystallites growth. The nucleation process, in all likelihood, involves sp3 and mixed sp3-sp2 carbon sites that give rise to subnanocrystalline carbon structures as the first step in carbon build-up on the surface of plasma-generated carbons. The nucleation and growth mechanism allows explaining the difference in kinetics of methane decomposition over different types of carbons.
Methane Activation Acknowledgment. Support from NASA, Glenn Research Center is greatly acknowledged. The authors thank Dr. Ali T-Raissi for fruitful discussions. The assistance from Kirk Scammon, Drs. Nina Orlovskaya, and Nahid Mohajeri is also appreciated. References and Notes (1) Choudhary, T.; Aksoylu, E.; Goodman, D. W. Catal. ReV. 2003, 45, 151. (2) Steinberg, M. Int. J. Hydrogen Energy 1999, 24, 771. (3) Muradov, N. Int. J. Hydrogen Energy 1993, 18, 211. (4) Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed.; John Wiley & Sons: NY, 1992; Vol. 4, p 631. (5) Lynum, S.; Hildrum, R.; Hox, K.; Hugdahl, J. Proc. 12th World Hydrogen Energy Conf., Buenos Aires, Argentina, 1998; p 637. (6) Muradov, N. In Hydrogen Fuel: Production, Storage, Utilization; Gupta, R., Ed.; CRC Press: Boca Raton, FL, 2008; p 33. (7) Muradov, N. Catal. Commun. 2001, 2, 89. (8) Muradov, N. Int. J. Hydrogen Energy 2001, 26, 1165. (9) Moliner, R.; Suelves, I.; Lazaro, M.; Moreno, O. Int. J. Hydrogen Energy 2005, 30, 293. (10) Czernichowski, A.; Czernichowski, P.; Ranaivosolarimanana, A. Proc. 11th World Hydrogen Energy Conf.; Stuttgart, Germany, 1996; p 661. (11) Bromberg, L.; Cohn, D.; Rabinovich, A.; O’Brien, C.; Hochgreb, S. Energy Fuels 1998, 12, 11. (12) Gonzales-Aguilar, J.; Moreno, M.; Fulcheri, L. J. Phys. D: Appl. Phys. 2007, 40, 2361. (13) Muradov, N.; Smith, F.; T-Raissi, A. Catal. Today 2005, 102103, 225.
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